1. Technical Field
The present disclosure relates to a semiconductor memory device, and more particularly, to a flash memory device and a voltage generating circuit for the same.
2. Discussion of the Related Art
Semiconductor memory is most commonly used in the design of microprocessor based digital logic and can be found in a wide variety of technologies ranging from satellites to consumer electronics. Therefore, advances in the technology for fabricating semiconductor memory with high integration (information storing capacity) and high speed performance are of great importance.
Semiconductor memory devices are mainly classified into volatile memory devices and nonvolatile memory devices. A volatile memory device writes and reads data while power is applied thereto, whereas the data is lost when the power is turned off. On the contrary, a nonvolatile memory device such as a mask read-only memory (mask ROM, MROM), a programmable ROM (PROM), an erasable and programmable ROM (EPROM), an electrically erasable and programmable EPROM (EEPROM), and so forth, can retain the data when the power is turned off.
The data storage state of a nonvolatile memory device may be permanent or reprogrammable according to the fabrication technology. Among nonvolatile memory devices, the memory contents of MROM, PROM and EPROM are not easily modified by users. On the contrary, since the memory contents of an EEPROM can be electrically erased and modified, EEPROMs are increasingly applied to systems that require continuous update. In particular, it is very advantageous to apply a flash EEPROM (hereinafter, referred to as a flash memory) to a high capacity auxiliary memory device because its integrity is higher than a conventional EEPROM.
Flash memory may be classified into a NOR type and a NAND type according to the connection state between a cell and a bit line. NOR flash memory is configured such that two or more cell transistors are connected to one bit line in parallel. NOR flash memory stores data using channel hot electrons, and erases the data using Fowler-Nordheim (F-N) tunneling. NAND flash memory is configured such that two or more cell transistors are connected to one bit line in series. NAND flash memory writes and erases data using the F-N tunneling. Although NOR flash memory with high information storage capacity uses large amounts of current, it can easily handle high speeds. In recent years, the information storage capacity of NOR flash memory has been great improved by employing a multi level cell (MLC).
When a single-bit of data is written to flash memory, the data stored in a unit cell may be represented as two threshold voltage distributions corresponding to data ‘1’ and ‘0’. On the contrary, when multiple bits of data are written to flash memory, the data stored in a unit cell may be represented as four threshold voltage distributions corresponding to data ‘11’, ‘10’, ‘00’ and ‘01’, respectively. As the number of data bits stored in each cell increases, a greater number of voltage levels are required for programming, erasing, and reading out the respective data. Therefore, it is important to be able to generate each voltage more accurately and precisely to program, erase, and read out data stored in flash memory. There is also a need for flash memory that can maintain the levels of each generated voltage without fluctuation.